Flavonoids are a diverse group of phytochemicals with 15 carbon skeleton arranged into two rings connected by three-carbon bridge. More than 10,000 flavonoids have been isolated and identified in plants. They are classified on the basis of degree of oxidation/saturation at various positions of central ring (ring C) and overall hydroxylation patterns. 9Different flavonoids include anthocyanins, flavones, flavonols, isoflavones and flavanones. Diverse functional roles of flavonoids have been critically studied and well-accepted now. Some of them include flower coloration, UV filtration, defence, chemical messengers, physiological regulators and are increasingly becoming the subject of medical research due to their potential health benefits. Biosynthetically, flavonoids are formed via two synthetic pathways: shikimic acid pathway or phenylpropanoid pathway and malonic acid pathway or polyketide pathway. Biologically active flavonoids span various orders of magnitude and the most important therapeutic uses of flavonoids include antioxidant, anticancer, anti-inflammatory, cardioprotective and antimicrobial activities. In this chapter, we focus mainly on structure and biosynthesis of flavonoids, along with their therapeutic applications.